The two students accused of opening fire at the STEM School in a Denver-area suburb, killing one classmate and injuring eight others May 7, were charged with murder and multiple counts of attempted murder Wednesday.

The suspects, Devon Erickson, 18, and Maya McKinney, a 16-year-old transgender student who goes by the name Alec, were formally charged in court with more than a dozen counts, including first-degree murder, attempted murder, theft, arson, and possession of weapons on school grounds.

Prosecutors charged the 16-year-old suspect as an adult. However, his attorneys asked the judge to set a hearing to move the case back to juvenile court, a spokesperson for the court told BuzzFeed News.

Both suspects were held without bond. They were arrested after authorities say they went deep inside STEM School Highlands Ranch and began shooting at classmates at separate locations.

Erickson appeared in court Wednesday with glasses and pink-streaked hair.

Neither suspect was on law enforcement's radar prior to the shooting, Douglas County Sheriff Tony Spurlock has said, adding that they used at least two handguns in the attack.

District Attorney George Brauchler said investigators know where the guns came from but has declined to comment on how they were obtained.

People under the age of 21 are not allowed to possess or buy handguns in Colorado.

Prosecutors have not provided a motive for the shooting.

Instagram accounts that appear to belong to the suspects show them pictured with friends and at concerts.

Instagram

Instagram

Last month, the 16-year-old suspect posted a series of photos from prom, including one of the two suspects with two other boys.

In a post from October 2018, he wrote, "I'm only posting one of my senior pictures so people don't think I dropped out since I don't take classes at stem anymore."

Erickson's former friend and classmate, Kevin Cole, told CNN that he used to frequently bully other kids and joke about mass shootings.

Cole said he would enter the classroom and say, "when the pencil hits the floor I'm going to start shooting," after which he would drop pencils during the class.

Cole also described threats he would allegedly make.

"He would just get really close and kind of hunch himself over your shoulder as you were sitting down, and he would just whisper in your ear, 'Don't come to school tomorrow,' and just kind of crack a smile and walk back to his seat," Cole told CNN. He added that people would dismiss it as "that was just a Devon thing" and laugh it off.

A tweet in January from an account that appears to belong to the 16-year-old suspect said, "FUCK stem kids i swear to fucking god."

Twitter

Other tweets referenced him missing his father, who according to reports has repeatedly entered the country illegally and has had run-ins with the law.

While school shootings have become all but ubiquitous in the US, it's rare for the attacks to be carried out by more than one suspect.

The most notable case involving more than one shooter was at Columbine High School, where 13 people were killed by two students on April 20, 1999. STEM School is located less than 8 miles from Columbine.

When asked by reporters about whether investigators believed there was a connection between the STEM and Columbine shootings, Brauchler declined to comment.